There have been attempts made to manage color in processes that include dispensing colorants. For example, the International Color Consortium (ICC) specifications (e.g., ICC.1:2010 version 4.3.0.0) for cross-platform color management systems are widely used in some color related industries such as publishing. The ICC specification is referred to in many international and other standards to address challenges in color management, for example, when printing the same document may look different when printed on different printers, viewed on different monitors, or viewed under different quality of lightings. Such challenges can be caused or aggravated by a variety of factors including different device color gamuts (e.g., range of realizable colors), light source metamerism and observer vision metamerism. To address these problems, colorimetric and/or spectral transformation can be used to map the colors from one device color space (e.g., source device) to another device color space (e.g., destination device).
The ICC has adopted a device-independent color transformation. For each device, there is a transformation from that device to a standard color space, e.g., the International Commission on Illumination (CIE) standard color space CIEXYZ/CIELAB (or XYZ/L*a*b*). Transformations have source-to-standard color space or destination-to-standard color space information.
In a typical ICC workflow, the transforms from device to the standard color space (i.e. profile connection space (PCS)) are embedded in the ICC Source Profile or ICC Destination Profile. An ICC profile may describe unique color characteristics of a device and may contain a transform from a device to the PCS. ICC profiles may be developed for devices such as input devices (e.g., scanners, digital cameras, or the like), output devices (e.g., printers, film recorders, or the like), and display devices (e.g., LCD monitors, projectors, or the like).
For example, display calibration and characterization (or profiling) can be an important aspect of a color managed workflow. Calibration is a process whereby a device is brought to a standard state (e.g., a color temperature of 6500K and gamma of 2.2) and the device is characterized by determining how the monitor represents or reproduces color. The monitor can be characterized, for example, by measuring how the monitor displays known color values. An ICC profile can then be created that defines a quantitative relationship between device dependent RGB values and the CIE device independent XYZ/L*a*b* values. In a similar manner, an ICC profile for a CMYK printer can provide a quantitative relationship between device dependent CMYK values and the CIE device independent XYZ/L*a*b* values. Such relationships may be established with a Look Up Table (LUT) to which interpolation is applied that are pre-calculated or the use of color matrices to carry out color transformations on the fly via the Profile Connection Space (PCS) based on a CIE standard color space (XYZ/L*a*b*).
In another example, a color transformation can define the transform from an RGB digital camera input device to a CMYK inkjet output device via the PCS based on a CIE standard color space (XYZ/L*a*b*). A printer profile can be generated by printing out a series of patches and then measure them with a color measuring device. In this way, the printer profile can be used to convert a CMYK device color value into the PCS XYZ/L*a*b* value, and vice versa.
In order to perform a transform from the camera RGB values into CMYK values for the printer, there may be two tables (A to B and B to A) needed. For example, in an A to B conversion, an image profile embedded with a digital camera profile allows the conversion from device color (RGB) to CIE color specification (L*a*b*). On the other hand, in a B to A conversion, the converted CIE color specification (L*a*b*) can be converted into a display device color specification using a color monitor profile (e.g. RGB profile) which can be created via a monitor calibration color matrix. Alternatively, the vision color can be converted into an inkjet printer device color (e.g., CYMK) using the inkjet printer profile. During the process in B to A conversion, some source colors may lie beyond the destination device gamut, so those PCS colors will be compressed into the device gamut, which can result in a difference between the converted device colors and the source CIE color specifications. This is known as the color gamut mapping process.
The ICC cross platform color management system was approved as an International Standard, ISO 15076-1:2010 “Image technology color management—Architecture, profile format and data structure”. ISO 15076-1:2010 specifies a color profile format and describes the architecture within which it can operate. This architecture supports the exchange of information which specifies the intended color image processing of digital data. The required reference color spaces and the data structures (tags) are also specified.
The ICC cross platform color management system may be subject to one or more challenges, problems or limitations. For example, color matching accuracy may be a challenge as the inherent method and algorithms that manage this translation and interpretation can lead to inaccuracies that may be unacceptable in some industries, such as textile printing, for example. A properly managed ICC color workflow may specify that as long as 80% of the colors produced are within a delta error (DE) tolerance of 3-6 DE with not more than 3% exceeding a 12 DE, the process is within tolerance. Delta Error (DE) is a numeric description of the amount of color error that a produced color is from the desired or “standard” color, e.g., CMC(l:c) Colour Difference Formula. There are industry accepted formulas for calculating the DE value between 2 colors. In the textile industry the normal maximum tolerance is 1.5 units of DE with the normal deviation being a “1” or less. Some products may require a color accuracy of 0.5 DE units or less. It may be difficult or even impossible to achieve color accuracies in this range within an ICC color managed workflow. One reason for this limitation may be that the ICC color managed workflow was not intended to achieve the level of color accuracy that is often required in the textile industry. The ICC color managed workflow may have been designed for the aesthetically pleasing and somewhat color accurate reproduction of photo realistic images (e.g., images commonly used in graphic arts). It may not have been designed for the accurate reproduction of “spot” color as the textile industry requires.
In addition, in a traditional ICC managed color workflow, users may have limited options for controlling the accuracy of the color. Indeed, the user may only be capable of controlling the accuracy of the color by manipulating the image itself. In other words, the data file itself must be adjusted or “shifted” to get the output color closer to the tolerance that is required. By doing this, the on screen representation may change and the user may no longer have an accurate representation of the final desired product. This may lead to extreme difficulties during the color development and matching process.
Another limitation of the ICC workflow involves the default ICC illuminant. By default, the ICC color managed system works in a D50 illuminated workflow. D50 is the CIE Standard Illuminant at a co-related color temperature of 5000K with defined spectral power distribution that is used to view the final product under. D50 is a commonly used and accepted industry illuminant for graphic arts. A textile industry standard for daylight is D65, but in the printing industry and graphic arts areas, D50 is commonly used as the daylight standard. For example, the textile industry has adopted the D65 illuminant as a common standard. By definition, the CIE and ISO state that the “CIE standard illuminant D65 should be used in all colorimetric calculations requiring representative daylight, unless there are specific reasons for using a different illuminant.” (ISO 11664-2:2007(E)/CIE S 014-2/E:2006). Though mathematical models have been developed to allow D50 profiles to be used in a D65 environment, these mathematical conversion models may be themselves prone to introducing errors and may be, in worst case scenarios, approximations of the correct values. This can lead to further inaccuracies in the produced color that may be unacceptable in the textile supply chain.
Yet another limitation of the ICC workflow involves profile chart accuracy. In particular, this limitation is associated with the “profile chart” that is typically printed to create a printer color profile. This involves the physical printing of industry defined color patches that represent the permutations of the colorants (e.g., dyes/inks) that are available in the print machine. This chart normally encompasses from 800 to 2000 unique colors that the user prints. These color patches are created by software sending unique values to the printer. The printed patches are then measured with a spectrophotometer. By doing this, a look up table (LUT) can be created that correlates the “raw” device values sent to the printer to a known color value. The profiling software then will take these related values and populate a 3 dimensional color space that is representative of the printer. A potential problem with this method can be that the color space that the printer is capable of representing can potentially include a large number (e.g., millions) of colors and therefore the color engine that uses the profile must “interpolate” between the known colors from the profile chart to predict the values for an unknown color. This interpolation may be inaccurate at its best and its accuracy may be heavily dependent on the number of patches printed, the number of primaries (e.g., colorants, inks, dyes or pigments) in the digital color dispensing machine, and the accuracy of the color readings taken by the operator. One way to increase the accuracy of this process is to increase the number of color patches that are utilized in the three dimensional color space. This can presently be achieved by printing a much higher number of patches on the profile chart. This may not be practical as the time required to accurately measure the number of required patches and the accuracy of these additional readings may make the profiling process even less accurate.
Still another limitation involves metamerism. In colorimetry, metamerism refers to a pair of objects having different spectral reflectance curves but the same colorimetric specification (i.e. color match) for a given set of conditions. If, upon a change in illumination, the pair of objects no longer match, then the pair are said to exhibit illuminant/source metamerism. If upon a change in observer, the pair of objects no longer match, then the pair are said to exhibit observer metamerism. A spectral power distribution describes the proportion of total light emitted, transmitted, or reflected by a color sample at every visible wavelength; it precisely defines the light from any physical stimulus. However, the human eye contains only three color receptors (i.e., three types of cone cells), which means that all colors are reduced to three sensory quantities, called the tristimulus values (X, Y, Z). Metamerism occurs because each type of cone responds to the cumulative energy from a broad range of wavelengths, so that different combinations of light across all wavelengths can produce an equivalent receptor response and the same tristimulus values or color sensation.
It is not unusual for the textile industry to require color matching to be done under different illuminants simultaneously. This is to allow a sample to meet the color matching requirements to a standard color as it is moved through the supply chain and retail environments with a wide variety of illuminations with different lighting qualities. This type of color matching is called non-metameric color matches. This capability may be difficult or impossible under any circumstance achievable in an ICC color managed workflow since the ICC workflow is based on colorimetric quantities as opposed to spectral quantities.
Another limitation relates to visible wavelength range. Because the ICC platform is built on the colorimetry foundation, the applications of the ICC workflow may be restricted to the visible wavelength range. In other words, useful industrial applications in the ultraviolet and infrared wavelength ranges, such as in military camouflage applications, may not be applied.
In some conventional color matching operations for color dispensing systems one or more turbid medium theory models may be used for calculating optical properties (e.g., absorption and scattering coefficients) of materials (e.g., substrate and colorants). The Kubelka-Munk theory is one widely used for turbid medium models.
There may be one or more challenges, problems or limitations with conventional applications of turbid medium theory models, including Kubelka-Munk, in color matching for digital colorant dispensing systems. For example, some turbid medium theory models may involve relatively complex mathematical operations and may require substantial computation capacity or time to carry out the mathematical operations. Turbid medium theory models have been applied to certain industries (e.g., textiles) in a conventional manner on a “color by color” basis and may be restricted to less than 20 colors per design colorway for colorant dispensing operations. It may not be practical or economically feasible (e.g., in terms of computation time) to perform color matching for each pixel of a print image being configured for digital colorant dispensing using a turbid medium theory model, such as Kubelka-Munk, in a conventional manner.
Another potential limitation of some turbid medium theory models relates to “fall on colors.” Fall on color refers to color characteristics of colorants dispensed in layers in which a color of one layer may be applied over (or fall on) a color of another layer resulting in a color having properties different than either of the individual layer colors. Some turbid medium theory models may have no practical method to predict the resulting color of multiple colors falling on top of each other as may occur in a typical color dispensing operation (e.g., a textile printing operation) where color separated print colors are applied one by one in sequence (e.g., via a corresponding print screen) in which colors may or may not overlap each other.
Another potential limitation of some turbid medium theory models may be that some of the models offer no practical method to predict tonal colors, in a color consistent manner, for the potentially large number of pixels for a typical print with tonal design. In addition, some of the turbid theory medium models may offer no practical method to predict a hue shift that may occur as part of tonal development.
In yet another potential limitation, some turbid medium theory models may provide little or no way to address matching a color target that is outside a color gamut limit of a colorant database. In some conventional colorant dispensing environments, there may be multiple colorant dispensers that are the same or different from each other. There may be color differences in the colors produced by each colorant dispenser.
Some implementations were conceived in light of the above-mentioned challenges, problems and limitations, among other things.